In fact this description is more appropriate than the author can have imagined because the major feature of the rabbit retina is now known to be the 'visual streak', an extended area centralis which receives the image of the animal 's visual horizon. In the subsequent pages I have brought together a collection of observations which reveal the extent of the interdependence between the special:ised eye of the rabbit and the organisation of its central nervous system. The work presented here is thus unified by the theme of the topographical relationship between the visual field and its representation in the central nervous system, both in the overt mapping sense and in more subtle ways. My use of this theme stems from the influence of Professor D. WHITTERID~E who has emphasised the importance of mapping studies (WHITTERmGE, 1965); some of the material is a continuation of work begun by WHITTERIDaE and his co-workers (CHOUDHURY & WH1TTERIDGE, 1964; 1965; KERR & SENEVIRATNE, 1963). Other components, including most of the single unit studies, were carried out some years ago under the supervision of Professor WHITTERIDGE at Edinburgh University. One initial problem faced in carrying out such a topographical study is that

Topographical relationships between the anatomy and physiology of the rabbit visual system

In psychophysical experiments described previously we found that a stimulus surrounding a retinal scotoma fills in the scotoma instantaneously with its brightness and color. However, it takes a few seconds to fill in a black stabilized disk which is equally immobile with respect to the retina. An experiment carried out to investigate the cause of this discrepancy suggested that a presumed contrast firing of neurons along the inside border of the black stabilized disk may explain the phenomena. The postulated border contrast discharge may form a barrier for the spreading of brightness information from the surround. If no barrier is generated (as in retinal scotomata) or if it has disappeared (by stabilization) the spreading of the activity itself occurs very rapidly (probably in higher centers). The co-operation of these effects, the activity along the border of a field (the simultaneous contrast) and the filling-in, investigated in the experiments, may explain many perceptual phenomena. A model is proposed which is based upon stabilization experiments, neuronal recordings of other authors and reports from patients with defective visual systems. The well-known first stages (retina-lateral geniculate body-area 17) and a following, yet unlocalized, stage (called “higher center”) are distinguished. Stabilization experiments suggest a spreading capacity and a long adaptation time of this higher center. The spreading of activity is inferred clinically and from observation of the filling-in phenomenon. Long adaptation time is suggested by the slow disappearance of the stabilized image. In normal vision small eye movements, continually activating on-center cells and off-center cells along the contrast border preserve continuous perception of the whole homogeneously illuminated area from transient border activities. Neuronal mechanisms and a model of antagonistic barriers and mutual neutralization are discussed and indicated in a diagram.

Simultaneous contrast, filling-in process and information processing in man's visual system.

Lateral interactions in the outer retina

The synaptic contacts made by carp retinal neurons were studied with electron microscopic techniques. Three kinds of contacts are described: (1) a conventional synapse in which an accumulation of agranular vesicles is found on the presynaptic side along with membrane densification of both pre- and postsynaptic elements; (2) a ribbon synapse in which a presynaptic ribbon surrounded by a halo of agranular vesicles faces two postsynaptic elements; and (3) close apposition of plasma membranes without any vesicle accumulation or membrane densification.

Synaptic relationships in the plexiform layers of carp retina.

Information about the environment is received by animals via a number of sensory structures, each specialized to respond optimally to a particular physical or chemical change in the environment. Of these structures, photoreceptors are of common occurrence, and mediate information that is of primary significance to the life of the organism and the species. Photoreceptors function by means of pigments that absorb light in the near ultraviolet and visible regions of the spectrum, i.e., roughly 300 to 800nm. The lateral eyes of vertebrates possess such photopigments located intracellularly in specialized segments of the retinal receptors. In these outer segments light quanta are absorbed, and after a transduction and amplification process — about which little is known — excitation of the retinal neurones occurs, leading to impulses that proceed out of the retina to the visual centers of the brain. This chapter is concerned with the vertebrate visual cells and the photopigments contained therein. It deals with the manifold types of visual receptors throughout the various vertebrate groups, and the possible biological significance of this multiplicity. It deals similarly with the visual pigments within these receptors and attempts to sort out some ecological and phylogenetic relations suggested by the properties and distribution of these photopigments. Because of the manner in which this volume is organized, this chapter has a limited scope, being concerned mainly with those pigments that have been extracted out of the retina by appropriate solvents, chiefly digitonin solution.

C R Michael

Papers

Receptive fields of single optic nerve fibers in a mammal with an all-cone retina. I: contrast-sensitive units.